In PCM systems, signals can be conducted from and to a transmission unit either e.g. on an E1 line in accordance with the European standard or e.g. on a T1 line in accordance with the American standard. The E1 line can be e.g. a 75 .OMEGA. coaxial cable or a 120 .OMEGA. twisted pair cable, while the T1 line may be e.g. a 100 .OMEGA. pair cable. Earlier, the circuit board of the transmission unit was only provided to be compatible either with an E1 or with a T1 line, but nowadays the IC circuits on a transmission card are suitable for use both with E1 and T1 lines, whereby both these lines can be wired to the circuit board and either line may be chosen for electric connection to the transmission circuit.
In the transmission unit of a PCM system, e.g. in the transmitter/receiver transmission unit of a Pan-European GSM mobile communications system, the transmission circuit board must be matched to the characteristic impedance of the transmission cable used to bring about the desired signal and impedance levels. For example, cables of the E1 line or T1 line mentioned above may be used as the transmission line. For matching of different line impedances of a transmission card which is as versatile as possible, it is known to use e.g. a transformer match as the one shown in FIG. 1 in the appended drawing, the primary coil having a center tap that is connected to an operating voltage +V, FIG. 1 illustrates an impedance matching line controller circuit having a control stage 5 for controlling the transmission of a cable, resistors R and a transformer having primary and secondary coils where there is an intermediate output in the secondary coil, whereby a suitable output resistance R of the primary coil is set at different transformation ratios of the transformer T120, T75, TCOMM, for example, a 75 .OMEGA. output resistance T75 is set at a transformation ratio of 1:1.26 to be suitable for a 120 .OMEGA. cable or at a transformation ratio of 1:1.15 for a 100 .OMEGA. cable. It is a problem with this implementation that a big transformer is required.
It is known from patent publication FI-95182 to use a line controller circuit implemented without any separate intermediate output in the secondary coil of the transformer, as shown in FIG. 2 of the appended drawing having similar components to FIG. 1, for the impedance match between the intermediate output circuit and the transmission line. In the line controller circuit of the publication, the output impedances R.sub.opt and the transformer's numbers of turns n.sub.opt, are matched so that the circuit will provide simultaneously the signal level required by two transmission lines each having a different impedance. It is a problem with state-of-the-art impedance matches that it is usually possible to match them simultaneously to be suitable only for two different characteristic transmission line impedances.